Nope, what you see in steemit is being "filtered" through a User Interface (called "condenser", for obscure reasons). On the blockchain you have EVERYTHING. You have your original post and then you have a subsequent "transaction" with the modifications you've done. When the "condenser" prepares the web page to display it to you, it combines all the transactions that refer to a post and "merges" them to show you the latest version of a post.

So what you see in a web page is a merged version of the original post and all the subsequent modifications. Blockchain is not changed, it's "appended to"

Here is a picture of a comment I've written several minutes ago:

And then I did some stuff and came back and EDITED that same comment. This edit is another transaction on the blockchain which looks like this:

But when you read the comment on Steemit you only see the MERGED version from the two transactions, which corresponds to the FINAL version.

The rule of 7 days is probably to protect the blockchain from too many "edit" operations maybe (in relationship with old content) ? And to keep coherent with the rewards cycle of 7 days ? Just speculating here, I'm not sure